It is generally known that gas discharge lamps, for example the well-known TL-lamps, are driven by an electro magnetic ballast (EM ballast). FIG. 1 is a schematic block diagram, illustrating such conventional EM ballast 1 for a lamp 2. The ballast 1 of this example comprises an inductor L and a capacitor C in series with the lamp 2 to be driven, and a mechanical switch S in parallel to the lamp, typically of a bimetal design. The ballast 1 further has input terminals 3 for connection to mains, typically 230 V 50 Hz in Europe. Lamp connector terminals are indicated at 4, lamp electrodes are indicated at 5. In the case of such conventional ballast, the lamp can only be switched ON and OFF by switching the mains.
In a more sophisticated design, the mechanical switch is replaced by a controllable semiconductor switch, operated by an intelligent control device such as for instance a micro controller. FIG. 2 is a schematic block diagram, illustrating such ballast 10. Compared to the example of FIG. 1, the mechanical switch S has been replaced by an electronic switching circuit 20. This electronic switching circuit 20 comprises a full-wave rectifier 21 (shown as a four-diode bridge) having input terminals 22, 23 connected in parallel to the lamp 2, and having a positive output terminal 24 and a negative output terminal 25. The electronic switching circuit 20 further comprises a semiconductor switch 26, shown as a MOSFET, connected between the positive and negative terminals 24, 25.
The electronic switching circuit 20 further comprises a control device 28, having a control output 28a connected to the control terminal of the switch 26. The control device 28 may derive its power from the terminals 24, 25, or may derive its power from an external circuit (not shown). The control device 28 may be responsive to external command signals, transmitted over an external circuit (not shown), via a wired or wireless link, e.g. RF.
Assume that the mains power is switched on while the switch 26 is OFF, i.e. non-conductive. The voltage from the mains is insufficient to start the lamp. Starting the lamp is done by the controller 28 in two steps. The first step involves switching the switch 26 ON, i.e. generating a control signal Sc for the switch 26 such as to render the switch 26 conductive. Now, an AC current will flow through the inductor L and the lamp electrodes 5, heating the lamp electrodes 5. In a second step, the controller 28 switches the switch 26 OFF again, i.e. it generates its control signal Sc for the switch 26 such as to render the switch 26 non-conductive. As a result of this interruption, the inductor L develops a high voltage causing breakdown and ignition of the lamp, so that lamp current flows between the electrodes 5 within the lamp.
The magnitude of the ignition voltage induced by the inductor L depends on the amount of energy E(L) stored in the inductor at the moment of interrupting the current circuit, which can be expressed as E(L)=0.5·L·I2.